<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
  /*  h3{
        color: red
    }  */
</style>
<body>
    <h3>我是标题</h3>
   <script>
    /* var age=20;
     console.log(age) */

     alert("我是弹出框 ");
     document.write("要输出的内容");
   </script>
</body>
</html>